This invention relates to a process for producing small or minute capsules containing a water-immiscible material which comprises dissolving a first shell wall component in a water-immiscible material, which is the material to be encapsulated, dispersing the resulting mixture, said mixture being the oil or discontinuous phase liquid into an aqueous phase liquid containing an emulsifier which is an alkylated polyvinylpyrrolidone (PVP) polymer to form an oil-in-water (O/W) emulsion and thereafter adding a second shell wall component (usually dissolved in additional aqueous phase liquid) to the oil-in-water emulsion whereby the second shell wall component reacts with the first shell wall component to form a polycondensate shell wall about the water-immiscible material at the oil/water interface.
The process of microencapsulation described herein is a modification of known interfacial polycondensation techniques. Such techniques are thoroughly described in the literature, with the article entitled "Interfacial Polycondensation, a Versatile Method of Polymer Preparation" by P. W. Morgan, Society Plastics Engineers Journal 15, 485-495 (1959), providing a good summary of the reactions involved and the polymers which can be used in this method. The use of the technique of interfacial polymerization in a process of microencapsulation is also known; e.g., MICROCAPSULE PROCESSING AND TECHNOLOGY, Asaji Kondo, Edited by J. Wade Von Valkenburg, pp. 35-45, Marcel Dekker, Inc., New York, N.Y. 10016 (1979). Exemplary of the patents directed to microencapsulation via interfacial polycondensation reaction are U.S. Pat. Nos. 3,429,827, 3,577,515, and 4,280,833 and British Pat. No. 1,371,179.
Microencapsulation of water-immiscible materials utilizing an interfacial polycondensation reaction generally involves the following procedure. A first reactive monomeric or polymeric material(s) (first shell wall component) is dissolved in the material to be encapsulated to form the oil or discontinuous phase liquid. The discontinuous phase liquid is dispersed into an aqueous or continuous phase liquid to form an oil-in-water (O/W) emulsion. The continuous phase (aqueous) liquid may contain a second reactive monomeric or polymeric material (second shell wall component) at the time the discontinuous phase is dispersed into the continuous phase. If this is the case, the first and second shell wall components will immediately begin to react at the O/W interface to form a polycondensate shell wall about the material to be encapsulated. However, the preferred practice is to form the O/W emulsion before the second shell wall component is added to the emulsion. This enhances the formation of a stable O/W emulsion before the interfacial polycondensation reaction is initiated and prevents the formation of agglomerates.
The capsules produced in this fashion may be any desired size, for example, of the order of 1 micron up to 100 microns or larger in diameter, preferably the size of the microcapsules will range from about 1 to about 50 microns in diameter. Capsules of this character have a variety of uses, as for containing dyes, inks, chemical agents, pharmaceuticals, flavoring materials, pesticides, herbicides, and the like. Any liquid, oil, meltable solid or solvent soluble material into which the first shell wall component can be dissolved and which is nonreactive with said first shell wall component may be encapsulated with this process. Once encapsulated, the liquid or other form is preserved until it is released by some means or instrumentality that breaks, crushes, melts, dissolves, or otherwise removes the capsule skin or until release by diffusion is effected under suitable conditions.
A method of microencapsulation based on in situ interfacial condensation polymerization is disclosed in British Pat. No. 1,371,179. This patent discloses a process which consists of dispersing an organic pesticide phase containing a polymethylene polyphenylisocyanate or toluene diisocyanate monomer into an aqueous phase. The wall forming reaction is initiated by heating the mixture to an elevated temperature at which point the isocyanate monomers are hydrolyzed at the interface to form amines, which in turn react with unhydrolyzed isocyanate monomers to form the polyurea microcapsule wall. One difficulty with this method is the possibility of continued reaction of monomer after packaging. Unless all monomer is reacted during the preparation, there will be continued hydrolysis of the isocyanate monomer with evolution of CO.sub.2, resulting in the development of pressure in the packaged formulation.
A method of encapsulation by interfacial condensation between direct-acting, complimentary reactants is disclosed in U.S. Pat. No. 3,577,515, which describes a continuous or batch method which requires a first reactant (shell wall component) and a second reactant (shell wall component) complimentary to the first reactant, with each reactant in separate phases, such that the first and second reactants react at the interface between the droplets to form encapsulated droplets. The process is applicable to a variety of polycondensation reactions, i.e., to many different pairs of reactants capable of interfacial condensation from respective carrier liquids to yield solid film at the liquid interface. The resulting capsule skin may be produced as a polyamide, polysulfonamide, polyester, polycarbonate, polyurethane, polyurea or mixtures of reactants in one or both phases so as to yield corresponding condensation copolymers. In the practice of the process described by U.S. Pat. No. 3,577,515, the liquid which preponderates becomes the continuous phase liquid. That is, in forming oil containing microcapsules, the aqueous liquid would preponderate; when water soluble materials are encapsulated, the oil phase would preponderate, i.e., become the continuous phase liquid.
Although there are a number of methods available in the art for producing microencapsules via interfacial polycondensation reactions, there are various disadvantages associated with the prior art methods. The encapsulated materials formed by the in situ interfacial polymerization process of British Pat. No. 1,371,179, require post-treatment to prevent continued carbon dioxide evolution and excessive caking, thereby increasing the costs of the finished product. The process described by U.S. Pat. No. 3,577,515, while adequate if one desires to encapsulate low concentrations of water-immiscible materials, is inadequate if concentrated amounts (i.e., greater than 480 grams/liter of water-immiscible material, is to be encapsulated in the respect that either one cannot form the necessary oil-in-water emulsion in the first instance or if microcapsules form, they cannot be maintained in discreet form since they tend to agglomerate into large unuseable masses.
U.S. Pat. No. 4,280,833 describes a process of microencapsulation via an interfacial polycondensation reaction whereby concentrated amounts of water-immiscible material, i.e., 480 grams or greater of water-immiscible material per liter of composition, is encapsulated in a polyurea shell wall with the finished capsules forming a suspension in the aqueous phase liquid. The ability to obtain high concentration microencapsulation is obtained by the use of the salts of lignin sulfonate to achieve exceptionally stable emulsions prior to the addition of the second shell wall component.
It has been discovered that an alkylated polyvinylpyrrolidone emulsifer can be used in the process of microencapsulation via interfacial polycondensation described herein to achieve high concentration microencapsulation. This is quite surprising since the unsubstituted polyvinylpyrrolidone polymer was not useful to achieve high concentration microencapsulation of water-immiscible materials. The present invention thus provides a new and improved encapsulation process via an interfacial polycondensation reaction which is rapid and effective to encapsulate high concentrations of water-immiscible material and which avoids the necessity of separation of the encapsulated material from the continuous, i.e., aqueous, phase liquid. Once the water-immiscible material, for example, a herbicide, is encapsulated, one has a solid in liquid suspension (i.e., a water-based flowable composition) which can be directly combined with other water-based materials, for example, pesticides or fertilizers.
The critical feature of the present invention resides in the use of the specific type of emulsifier described herein to form a sufficiently stable oil/water emulsion so that a concentrated amount of water-immiscible material is present in the water-immiscible phase and is thereafter encapsulated. Generally, there will be greater than 480 grams of water-immiscible material per liter of total composition. By use of the specific emulsifier described herein, it is possible to retain the finished microcapsules in the original aqueous solution, thus avoiding the additional step of separation of the microcapsules from the aqueous environment. Further, the finished microcapsules do not agglomerate nor does the aqueous capsule mass solidify when stored for extended periods of time (on the order of six months or greater) or when exposed for short terms to elevated temperatures.
The invention is applicable to a large variety of polycondensation reactions, i.e., to many different pairs of reactants capable of interfacial condensation at the organic/aqueous phase interface to form microcapsules. A number of basic types of polycondensation reactions, are known and can be utilized in the present process. Thus, as examples, the resulting capsule skin or enclosure may be produced as a polyamide, polysulfonamide, polyester, polycarbonate, polyurethane, or polyurea, and the reactions of the invention may also involve mixtures of reactants in one or both phases, so as to yield corresponding condensation copolymers if desired, e.g., mixed polyamide/polyester, or polyamide/polyurea capsule shell walls.
The present invention is particularly advantageous when employed to encapsulate agricultural chemicals, as for example, herbicides, especially acetanilide, acetamide, and thiocarbamate herbicides like alachlor, butachlor, metolachlor, acetochlor, .alpha.-chloro-N-(ethoxymethyl)-N-[2-methyl-6-(trifluoromethyl)phenyl]-ace tamide, triallate, diallate, and the like. Other types of agricultural chemicals which may be advantageously encapsulated according to this invention are insecticides, fungicides, plant growth regulators and herbicidal safeners (antidotes).
Aqueous suspensions of pesticide and herbicide microcapsules are particularly useful in controlled release pesticide formulations, because they can be diluted with water or liquid fertilizer and sprayed using conventional agricultural spraying equipment, thereby producing uniform field coverage of the pesticide or herbicide. Additives such as film forming agents can be added directly to the finished formulation to improve the adhesion of microcapules to foliage. In some cases, reduced toxicity and extended activity of encapsulated herbicides and pesticides may result.
Experiments indicate that conventional oil/water herbicide emulsifiers fail to produce suitable emulsions for attaining microencapsulation of concentrated amounts of herbicide material and avoiding solidification of the oil/water mass when the second shell wall is added to the oil/water emulsion. Additionally, attempts to encapsulate concentrated amounts of acetanilide and thiocarbamate herbicides (four to five pounds per gallon) using traditional interfacial polymerization techniques, as for example that disclosed in U.S. Pat. No. 3,577,515, have resulted in unsatisfactory formulations because of the problem of rapid herbicide crystal growth in the finished suspension, as well as agglomeration or solidification of the microcapsules in the finished suspensions. The problem is particularly acute with the acetanilide/acetamide herbicides. Crystal growth is undesirable because once it occurs past a certain level, the final formulations cannot be used directly; rather the microcapsules must be separated from the aqueous solution and resuspended in water before they can be sprayed in conventional agricultural herbicide and fertilizer spraying apparatus.
It is accordingly a particular object of this invention to provide a process whereby greater than 480 grams per liter of acetanilide/acetamide herbicides, e.g., alachlor, butachlor, and thiocarbamate herbicides, e.g., triallate, diallate, and the like, is encapsulated in a polymeric shell wall with the finished microcapsules being suspended in the original aqueous phase liquid.